In the production of monolithic circuits, photosensitive resist materials play an important part. The use of such photoresist materials in circuit technology is based on their suitability in permitting the "engraving" of circuit patterns of specific dimensions in a predetermined monolithic substrate material, as for instance silicon. This is effected by means of a photolithographic process where, by photolithographic means, a two-dimensional pattern corresponding to the circuit design is first imaged on the photoresist coated substrate surface by means of a suitable exposure mask. Through a subsequent developing process, the desired resist patterns are obtained on the substrate surface. By means of suitable processes, such as coating or etching, specific profiles can be formed on the substrate surface. In these processes, the photoresist serves as protective varnish system for those areas of the substrate surface that have not been bared by the proceding photolithographic process.
According to their interaction with light, photoresist systems are classified into negative-working and positive-working systems. A negative-working photoresist is a photoresist which after exposure is insoluble in a suitable solvent, whereas the unexposed resist areas are dissolved by the developer. As a result bared, unprotected areas are obtained on the substrate surface which correspond to the opaque dark areas on the photomask. Examples of negative-working resist systems are photoresist materials based on partially cyclized cis-1,4-polyisoprene with a di-azidobenzalalkyl-cyclohexanone as a photoinitiator. In a positive-working resist system the photoresist is altered under exposure in such a manner that it is subsequently soluble in the developer. The exposed areas of the resist film are removed during developing, and the bared unprotected areas on the substrate surface correspond to the transparent areas on the photomask. Examples of positive-working resist systems are photoresist materials based on phenol formaldehyde resins (Novolak type) with a suitable molecular weight distribution, which contain a photoactive compound, a so-called inhibitor, for instance the substituted diazoquinones such as are described for example, in U.S. Pat. Nos. 3,046,118; 3,046,121; 3,106,465; and 3,201,239.
Compared with negative-working resist materials, positive-working resist materials show some advantages. Their sensitivity with respect to oxygen, for example, is much lower than that of negative-working resist materials which facilitates their handling in photolithographic processes. Positive-working photoresist materials show a higher resolution of the exposure geometries than negative-working resist materials. This is highly desirable considering the precision and the importance of details required for semiconductor purposes. Finally, masks of positive-working resist material which are for instance used for making diffusion windows in a silicon dioxide layer on semiconductor wafers, are much more easily removable with a solvent mixture from the substrate after etching the oxide layer, than masks made of negative-working resist materials.
A disadvantage of positive-working resist materials is, however, that compared with negative-working resist materials they adhere more easily to the contact exposure masks used for exposure, so that defects appear in the resist images. To give an example: with the use of a frequently employed positive-working resist, AZ 1350 J resist of Shipley Comp. Inc., a resist pick-up in the order of 20 to 30 was counted on a contact exposure mask after ten exposures. It has been suggested to coat the contact exposure mask with a fluorinated methacrylate polymer. Thus, the previously given resist pick-up by the contact exposure mask could be reduced from about 30 to about 0 to 7.
Other possible means of avoiding the resist pick-up by the mask were examined, one of them performs the image-wise exposure of a layer of positive-working resist material with a small gap in the order of 10 to 50 .mu. between mask and photoresist layer (proximity printing). Another version is the use of a projection exposure process, where via optical imaging, an image of the mask is produced on the photoresist-covered wafer.
It has been found that in spite of the off-contact exposure used in both the proximity and projection printing defects are still present due to missing portions of the photoresist layer. In layers of positive-working resist material blanket exposed without a mask, defects of the same type were found, too. Consequently, these defects must be caused by some other mechanism than by resist pick-up by contact with the exposure mask.
The cause of the defects is now believed to be an increased mechanical tension in the photoresist layer which is generated by molecular nitrogen released during exposure, and an interaction of the photoresist layer with the surface of the insulating layer on which it is coated owing to the electrostatic charging thereof.
It is therefore the object of the invention to provide a process for reducing the density of defects which are produced during the off-contact exposure of a positive-working resist layer consisting of a phenol-formaldehyde resin and an o-diazoquinone photoactive compound with actinic radiation due to the loss of photoresist particles.